This invention relates to a detonator system and to a time delay relay for use in a detonator system.
Different types of detonator systems exist. For example the use of shock tubes is well entrenched. Essentially a shock tube propagates an initiating signal at a predictable speed to detonators which are connected at intervals to the shock tube. The propagation speed and a designed pyrotechnic delay inside each detonator determine the duration of each time interval between the times at which the initiating signal is applied to one detonator and to a following detonator.
Although a shock tube system works effectively it does suffer from disadvantages. It is not easily possible to determine, precisely, the duration of the time delay between successive detonators. Another factor is that it is not possible to test the integrity of a shock tube detonator system without initiating the various detonators.
It is also known to make use of timing systems which are electrically based to determine the duration of each time delay. Electronic systems which are implemented through the use of integrated circuits are also in use. Generally however an electronically-based detonator system is relatively expensive and the cost thereof cannot easily be justified unless highly accurate time delay periods are required. A further factor is that the complexity of this type of system can generate user resistance.
With an electrically-based system connections between successive detonators are effected by means of elongate electrical conductors or wires. This type of system usually works at a low voltage and if a wire is damaged, an incident which can easily occur in the rough conditions which pertain at many blasting sites, electrical leakage can occur. This increases the potential unreliability of the system. If a high voltage could be used then the effect of electrical leakage would be reduced. However for safety and other reasons high voltages are not used, at least for testing purposes, in an electronically-based blast installation.
U.S. Pat. No. 4,445,435 describes a detonator blasting circuit which includes a capacitor which is charged by an input signal and which then powers a timing circuit to determine a time delay. Generally similar approaches are disclosed in U.S. Pat. Nos. 4,586,437, 4,712,477 and 5,602,360. In each instance the delay between each set of successive blasts is fixed only by the programmed time delays of the respective timing circuits. This means it is necessary to keep track of the geographical position of each detonator and once all the detonators have been placed in their respective boreholes, to program the appropriate time delays into the respective detonators.